1. Field of the Invention
The present invention relates to a seal for a regenerative heat exchanging system. More particularly, the present invention relates to a seal that is used to prevent leakage between a hot gas conduit and a cold air conduit of a regenerative air preheater.
2. Description of the Related Art
Regenerative heat exchangers are used to provide preheated air to heavy machinery, such as a fuel burning power plant. Such heat exchangers could be used with any type of machinery that exhausts hot gas and operates more efficiently when supplied with preheated air, such as, for example, chemical processors, refineries, pulp and paper mills, and ships. Typically, two fluid stream passages extend through the heat exchanger. The first passage is a hot gas conduit that communicates with a hot exhaust outlet of the power plant. Hot exhaust gases flow from the power plant exhaust into the hot gas conduit of the heat exchanger. The second passage is a cold air conduit that communicates with a cool air intake passage of the power plant. The cold air conduit feeds pressurized air into the intake passage of the power plant. As is known in the art, the regenerative heat exchanger extracts heat from the exhaust gases of the fuel burning power plant and transfers the heat to the cool air conduit, as described below.
One typical heat exchanger includes a movable heat exchanging body that moves between the hot gas conduit and the cool air conduit. The movable heat exchanging body cyclically collects heat from the hot gas conduit and releases the heat to the cold air conduit. In this manner, the heat from the power plant exhaust is used to warm the air that is being supplied via the intake conduit to the power plant. By supplying the power plant with preheated air, the efficiency of the power plant is improved. Additionally, the heat exchanger is environmentally friendly, as it recycles heat that would otherwise be exhausted into the earth's atmosphere.
The heat exchanging body is typically cylindrical in shape and is located in a sealed relationship within an outer housing of the heat exchanger. The heat exchanging body rotates about a center shaft within the housing of the heat exchanger. A plurality of radial walls extend radially outward from the center shaft and subdivide the heat exchanging body into angular sectors. As the heat exchanging body rotates, the angular sectors are alternately exposed to the hot and cold conduits of the heat exchanging apparatus. As an angular sector is exposed to the hot conduit, it absorbs heat from the exhaust gases of the power plant. The sector then rotatably moves and is exposed to the cold air conduit. The angular sector then releases heat into the cool air that is passed into the power plant intake.
Leakage between the hot gas conduit and the cold air conduit reduces the thermal efficiency of the heat exchanger. It is therefore desirable to seal each of the angular sectors from one another so that gas does not leak between the hot gas conduit and the cold air conduit. Toward this end, seals are mounted on the radial walls at the junctions between the movable heat exchanging body and the housing of the heat exchanging apparatus. These seals are typically mounted on the radial and axial ends of the radial walls. The effectiveness of the seals is extremely important, as a significant amount of the thermal efficiency of the rotary air heater depends on preventing leakage.
The previously described heat exchanger is often referred to as a Ljungstrom.TM. style preheater. As is known in the art, an alternative design is known as a Rothemuhle.RTM. style preheater which has a fixed heat exchanging body with movable conduits. Seals are also required between the fixed heat exchanging body and the movable conduits and suffer from the exact same problems described herein.
In particular, several difficulties are encountered in attempting to adequately seal between the heat exchanging body and the housing in which it is positioned. For example, the seals are typically exposed to harsh operating conditions, such as erosive fly ash and soot. As the heat exchanging body moves with respect to the housing or vice versa, the seals are also exposed to mechanical abuse because the seals are positioned to maintain sliding contact with the sealing surfaces. Consequently, the seals wear down quickly.
Further, the high operating temperatures of the heat exchanging apparatus expose the seals to thermal stresses which often cause the seals to warp. The high operating temperature also causes thermal distortions in the shape of the structural members of the heat exchanging apparatus, such as the housing and center shaft. The distortions in the shape of the seals and the structural members affects the clearance between the seals and the sealing surfaces, often resulting in leakage paths between the hot gas conduit and the cold air conduit. This reduces the thermal efficiency of the heat exchanging apparatus and also reduces the overall efficiency of the system.
Current seal designs do not adequately address these problems. Some seals are made from relatively thick metal which holds up well against corrosion and mechanical abuse. However, such seals are not very flexible and often lose contact with the sealing surface when the structural members of the heat exchanging apparatus thermally distort. Other seals are extremely flexible so that they initially offer better sealing characteristics by expanding or contracting when the structural members thermally distort to maintain contact with the sealing surface. However, such seals hold up poorly to corrosion and mechanical abuse.
Certain prior art seals have been equipped with flexible portions that allow the seal to flex in response to deformations in the heat exchanger. For instance, U.S. Pat. Nos. 3,977,465 and 3,703,206 each disclose seals having resilient portions that allow for straight line deformations. However, such seals are not configured to resiliently respond to the loads that are created by rotation of the casing of the heat exchanging body with respect to the outer housing of the heat exchanger.
There is therefore a need for an improved seal that may be used in conjunction with regenerative heat exchanging apparatuses to prevent gas leakage between the hot and cold conduits. The seal should hold up well against corrosion and mechanical abuse, but should also be flexible so that it maintains contact with the sealing surface even in light of the thermal distortions of the structural members in the heat exchanging apparatus. The seals should be manufactured of a material that is substantial enough to withstand the normal wear, corrosion and mechanical abuses that are typically associated with a regenerative heat exchanging apparatus.